Single-pressure SF.sub.6 circuit-interrupters are sensitive to the initial rate of rise of the recovery-voltage transient, which, in turn, is a function of the amplitude and frequency of the current interrupted di/dt.vertline.i=0.
The initial TRV (Transit Recovery Voltage) can be modified, or the maximum rate-of-rise of recovery voltage transient can be delayed, by adding shunting capacitance to ground, or across the interrupter. The time delay is essentially that of an R-C circuit, in which the "R" represents the surge impedance of the transmission line, or system, and the "C" is that of the added capacitance.
If the capacitance is added to ground, it is energized continuously, and must withstand all voltages at all times. Furthermore, final isolation from the system is made by a slow-acting disconnecting switch, which can produce transients resulting in excessive overvoltages, or false-relay operations. Also, the cost of capacitors may be prohibitive, if applied to both the bus side and to the line side of the circuit-breaker.
However, the capacitance can be added across the interrupter itself resulting in the following important advantages: The capacitor is not energized continuously, it does not affect relay operations when being switched by disconnecting switches, it modifies the initial recovery rate of both line and bus-side transients, it is closer coupled to the circuit-interrupter and therefore more effective, and is generally more economical.
There are also certain disadvantages associated with this type of capacitor application, i.e., the capacitor may couple excessive voltage across the open circuit-breaker, large amounts of capacitance may resonate with series circuit inductances, and the capacitor must withstand the open-circuit, low-frequency, one-minute voltage test.